1. Field of the Invention
The invention is concerned with techniques of collecting so-called insulating mineral fibers, particularly of glass fibers, with a view to separating, under the fiber making machines, the fibers and ambient gases--particularly induced gases or those used for drawing out the fibers--in order to manufacture a mineral wool mat.
2. Background of the Related Art
An important stage in manufacturing products based on mineral fibers such as glass fibers is their collection under the fiber making machines. This operation is intended specifically at separation of the fibers by air induction. This separation is carried out in a known manner by suction through a gas-permeable reception device impermeable to the fibers.
A standard type of collection device called a belt collector is described, for example, in U.S. Pat. 3,220,812 in which it is proposed to collect fibers from a series of fiber making machines on a single endless belt type conveyor permeable to gas and under which a vacuum chamber is placed, or better still several independent vacuum chambers. In this type of collection, the fiber making machines can be brought as close together as the respective limits of their sizes permit, which allows relatively short production lines. This point is fairly important considering that production lines can contain as many as 9 fiber machines or more, each fiber machine being around 600 mm in diameter, for example.
The bottom limit of product felt density is dictated primarily by problems of mechanical strength, which therefore allows manufacture of the lightest products possibly obtainable. However, obtaining heavy products poses many problems. The term heavy products is used to refer to products whose density is, for example, more than 2.5 kg/m2 in the case of glass wool products with fibers as small as 3 microns per 5 g, with the exception of dense products obtained by molding and pressing which do not come under the scope of this invention. This difficulty can easily be explained by the fact that the heavier the mat one attempts to produce, the greater the quantity of fibers deposited on a single surface area of the endless band, and therefore the greater the resistance to gas passage. To compensate for this reduced permeability, the negative pressure must be higher, which has the consequence of crushing the felt under pressure of the gases, such crushing being particularly noticeable at the bottom of the felt, i.e., the fibers collected first. Because of this, the mechanical performance of the product, particularly as regards regaining thickness after compression, is reduced. The resulting deterioration in quality is noticeable immediately when negative pressure is increased beyond 8000 to 9000 Pa, whereas in some installation a negative pressure of 10 000 Pa is necessary for mats with a density of 2500 g/m2.
To remedy this disadvantage, the gases may be drawn in only partially (i.e., only in certain areas) in order to limit negative pressure to a value which will not damage the felt, but there then occurs a phenomenon of fiber back flow in the direction of the fiber making machines for those areas not under suction. As well as being detrimental to good drawing out of the fibers, this back flow of gas causes an increase of temperature in the fiber making hood and thus a risk of pregellification of the binder: that is to say polymerization of the binder while the fibers are still separate filaments, which therefore virtually puts a stop to all its activity. In addition, this back flow can cause lumping, i.e., dense assemblies of conglomerated fibers harmful to the homogeneity and appearance of the product, and reduce its thermal resistance.
A reduction in speed of gas passage through the felts can be sought by spacing the fiber making machines apart from one another. However, any real gain is very slight since increasing the dimensions of the hood causes increased air induction and therefore an increase in the amount of air to be drawn out.
In European patent application EP-A-102 385, it was proposed to separate collection into two parts, each part receiving fibers produced by every other fiber making machine. In this case, collection is achieved by two conveyors facing one another in order to gather together the two half-felts formed. This type of collection has the advantage of providing products of good external appearance due to the presence on both faces of crusts glued together, which improves product mechanical strength. However, this collection device takes up more space than conventional collection devices and, for heavy densities, the binder sometimes polymerizes before the half-felts are brought together, thus causing the product to separate into layers.
This notion of sub-dividing the collection operations was set out elsewhere in U.S. Pat. No. 4,120,676 which proposes associating one collection unit with each fiber making machine, the production line thus being a juxtaposition of basic modules each producing a relatively thin felt, the different thin felts being later stacked to form a single very thick felt.
This modular design enables keeping fiber making conditions constant whatever the product being manufactured. However, the lightest products are therefore obtained with a production line used well under its theoretical capacity, which is not cost effective.
Another example of modularizing mineral wool production lines is the so-called drum-type collection devices in combination with a layer forming device. In this case, as shown in U.S. Pat. No. 2,785,728, reception occurs on drum type rotating parts. A low-density primitive is prepared by means of a collection device facing one or more fiber making machines, consisting of a pair of drums revolving in opposite directions whose perforated surface enables the gases to be drawn in by suitable devices located inside the drums. The primitive forms between the drums and falls down vertically before collection by the layer forming device, i.e., a pendular device which deposits the primitive in criss-cross layers onto a conveyor where the desired high density felt is obtained.
These modular collection device designs theoretically target a much wider range of products inasmuch as one always starts with a low density felt.
However, this requires a higher initial outlay with, in addition, a multiplication of the associated equipment (suction and washing devices in particular). Also, the means of separating the collection devices requires wide spacing of the fiber making machines, thus resulting in exceptionally long production lines as the number of fiber making machines is increased.
In addition, the likelihood of the product separating into layers and not being homogeneous limits the production of lower density felts. Thus, a lapping machine must have a primitive of at least 100 g/m2, below which its mechanical strength would be insufficient, particularly for withstanding the pendulum movements and a sufficient number of stacked layers, to obtain optimized distribution with the same number of layers at all points of the felt.
Also, always operating with the same yield of fibrous mass enables working in conditions encouraging the reproducibility of fiber making parameters and therefore their optimization, yet deprives the manufacturer of the fiber making machines the ability to process fibrous material at yields ranging, for example, from 1 to 10.
Lastly, for the same quality of fibers, a product of less density is marketed at a lower price. It would not appear very judicious to choose those conditions in which the production line produces the least tonnage.